How to gauge if someone is in love with you:

People who feel a connection naturally mimic each other. People who felt strongly about a romantic relationship they were in did not mimic an attractive, opposite-sex third party nearly as much:

Based on the recent literature indicating that nonconsciousbehavioral mimicry is partly goal directed, three studies examined,and supported, the hypothesis that people who are involved ina romantic relationship nonconsciously mimic an attractive opposite-sexother to a lesser extent than people not involved in a relationship.Moreover, Studies 2 and 3 revealed that romantically involvedpersons tended to mimic an attractive alternative less to theextent that they were more close to their current partner. Finally,Study 3 provided preliminary support for a potential underlyingmechanism, revealing that the effect of relationship statuson level of mimicry displayed toward an opposite-sex other ismediated by perceived attractiveness of the opposite-sex other.The present findings suggest that behavioral mimicry servesan implicit self-regulatory function in relationship maintenance.Implications for both the literature on relationship maintenanceand the literature on behavioral mimicry are discussed.

Source: “Mimicking Attractive Opposite-Sex Others: The Role of Romantic Relationship Status” from Personality and Social Psychology Bulletin